The attack on the health workers took place in the southwestern city of Quetta, while the drone strike targeted a militant hideout in the North Waziristan tribal region in northwest Pakistan.

Those killed in the attack on the outskirts of Quetta included three women, said a police spokesman Shahzada Farhat.

The vaccination team of three women and three men was waiting for a police escort when two gunmen opened fire on them, he said.

Local militants oppose the vaccination campaign, alleging that western governments use it as a cover to spy on insurgents. A Pakistani used the same method of a fake vaccination drive to collect evidence on al-Qaida's late chief Osama bin Laden, which led to his killing by a U.S. special forces team in a compound in the Pakistani city of Abbottabad.

Pakistan is one of only three countries in the world where polio, which can cause paralysis and death, remains endemic. The incidence of polio in Pakistan has hit a 15-year high; the World Health Organization has already registered 235 polio cases since January. Attempts to administer the vaccine have been hindered by militant attacks that have killed hundreds of health workers.

Meanwhile on Wednesday, a suspected U.S. drone strike targeted a compound in the North Waziristan tribal region along the Afghan border, killing four militants, three Pakistani intelligence officials said. They spoke anonymously because they were not authorized to brief the media.

The region has long been a safe haven for local and al-Qaida linked foreign militants who have been waging a war against the Pakistani state in a bid to enforce their own harsh brand of Islamic law.

The Pakistani army launched a major offensive in North Waziristan in mid-June, and claims to having killed over 1,200 militants so far.